Binary raster images represent image data as a sequence of pixels using a single bit per pixel. Binary raster image files may be contained in, for example, TIFF files or in PDL files such as, for example, PDF files. Using various lossless compression algorithms, binary image files may be relatively small, but are able to support high image quality. Print controllers may process such binary raster images at rates of hundreds of pages per minute. For these reasons, binary raster images are widely used throughout the printing industry. Currently, document creation and assembly systems obtain binary raster images in a variety of ways: i) capture by a scanning device; ii) output from a print controller decomposition service; and iii) import.
Regardless of their origin, all binary raster images are device-dependent. In other words, each binary raster image represents information in a way appropriate to a particular type of output device such as, for example, a printer to which the binary raster image is best used. The specific output device is identified as the target for the binary raster image, and the identity of the specific output device is part of the targeting of the binary raster image. When incorporated into documents such as Microsoft® Word, Microsoft® PowerPoint®, Adobe® PDFs or Xerox's® DigiPath Raster Document Objects™, the original targeting of the images remains.
Makeready/prepress applications running on client workstations may obtain device-dependent targeted raster images from different sources, e.g., by scanning physical pages into electronic files or by rasterizing pages represented in page description languages. Once on the client workstation, the targeted raster images may be submitted to a printer for printing.
There are two main types of device-dependent targeted raster images: binary raster images and device-dependent contone raster images.
Device-dependent binary rasters are printer specific. Binary raster images are primarily intended for use on traditional one-toner printers. Examples of binary raster image files include TIFF 6.0 CCITT Group 4 files and images stored in PDF files by various binary compression algorithms. As the characteristics of different print engines vary, so too do the device-dependent binary raster images that are targeted. Factors such as whether the engine writes white or black, the minimal reproducible line width, different tone reproduction curves, availability of particular half tone screens, and the use of error diffusion to minimize moiré effect all affect how best to render the raster data for a particular image.
Contone raster images in, for example, a CMYK print engine's color space, use multiple bits per pixel to represent their output. Formats may vary, but very commonly eight bits are used to represent each colorant. Hence, for a CMYK raster image consisting of four colorants (cyan, magenta, yellow and black), 32 bits are used to represent each pixel. However, while these CMYK raster images are well suited for a specific printer, if they are submitted to a different CMYK printer, image quality may be negatively impacted. In general, a contone raster image can become device-dependent when a color decomposer converts an output that is targeted for a CMYK printer to the CMYK color space, during which process any current rendering information is applied. The resulting decomposer output files then contain the CMYK data needed to target a specific output device. Examples of contone raster image files include TIFF 6.0 Technical Note #2 files, and contone raster images stored in Portable Document Format (PDF) files by a JPEG compression algorithms.
The above-described examples of raster image files may also be contained in Page Description Language (PDL) files. Examples of PDLs include Adobe Postscript, Adobe Portable Document Format (PDF) and Hewlett Packard's Page Composition Language (PCL). Moreover, when a PDL file contains a raster image that is targeted to a specific type of device, then the PDL file that contains the raster image also becomes targeted to the same type of device. However, there are many other ways in which PDL files may be made device-dependent, optimally targeted to particular output devices: i) varying rasterization parameters: printers have different resolution; ii) varying color spaces: color is a major source of PDL variations as producers generate files optimized for specific output devices; iii) varying halftone screening parameters: certain PDL files may contain screening instructions (such as the number of lines per inch and dot patterns) controlling the way in which the file is to be processed prior to printing; iv) trapping: to compensate for paper misregistration when processing PDLs for certain printers, abutting color areas are slightly spread or choked to avoid generating unwanted artifacts; v) varying Open Prepress Interface parameters: certain systems support replacing low resolution for position-only images with high resolution press images; vi) varying fonts: knowing font availability for different output devices determines where the PDL may be properly processed; vii) altering device specific PDL instructions: some PDLs may contain specific device control instructions associated with such document output functions as simplex versus duplex printing, document finishing, paper stock selection and covers; or viii) altering color separated files: during generation of color separated files, two processing techniques, undercolor removal (UCR) and gray component replacement (GCR), may be applied, and the resulting color separated files are generally targeted to specific printers.
For example, a specific PDL file format, PostScript, may be targeted to specific output devices via the following user controlled selections: i) effective raster image resolution; ii) color conversion strategy; iii) compression type and parameters; iv) anti-aliasing, which increases the number of bits per sample; v) font embedding; vi) black overprint; vii) Open Prepress Interface; viii) parse Document Structuring Convention (DSC) comments; and ix) autorotate pages.
In recent years, so-called preflight software tools or applications have been developed. Such applications evaluate files for print features that may cause possible defects (e.g., missing fonts, inappropriate raster image resolution, use of spot colors on a process color device, color spaces, and the like). Often users may select from among hundreds of possible criteria. Preflight software packages now form an established part of the printing industry and include offerings from a range of vendors.
Also, numerous current software applications (for example, Microsoft® Word®, Microsoft® PowerPoint® and Adobe® Acrobat®) allow users to create compound documents incorporating such raster images, where the term “compound document” simply is taken to refer to a document that contains one or more objects. The objects contained may include both include raster images and PDLs. The objects may generally be created separately from one another prior to their incorporation into the container compound document, thereby allowing the possibility that, even if the objects are of the same type, the objects may differ in certain crucial respects such as, for example, the intended target output device.